The new survey shows that levels of happiness are roughly constant across age groups, despite platitudes about the bliss of youth. Among older adults, happiness tended to vary little with gender or race, the survey also found.

Still, key factors separate the happy from the unhappy.

Unsurprisingly, people in excellent health were 25 percent more likely to say they were “very happy” compared to those in poorer health, the survey found. Feeling financially prepared for retirement and having good friends were associated with similarly increased odds that people would describe themselves as happy.

Happiness intrigues scientists. While it’s clear that good health contributes to one’s contentment, many researchers think the reverse is also true — that happiness leads to good health. But the mechanisms are not clear, said Marlene Lee, a senior research associate at the Population Reference Bureau, a nonprofit research organization.

“It’s just absolutely fascinating that your happiness can not only lead to your health, but also to other people’s happiness and their health,” Ms. Lee said.

Scientists strongly suspect that expectations and the ability to adapt to changing life circumstances also influence happiness. Ms. Lee said people who adjust their expectations as their social networks and lifestyles change may avoid feelings of loneliness and isolation, which are linked to illness and earlier death.As people age, their social networks shrink, scientists have found. But while having fewer social outlets may negatively influence health, the intensity and emotional connection of the remaining relationships increases with age, which may help compensate for fewer social ties.

Adapted by Personal Safety Nets® from “What Makes Us Happy” by Sarah Arnquist in the New York Times (June, 2009).